Process of electrodepositing chromium



Patented Sept. 26, 1933 UNITED STATES PATENT} OFFICE CHRO MIUM

Colin G. Fink, New York, N. Y., and Hugh David McLeese, Detroit, Mich.

No Drawing. Application January 14, 10:: Serial No. sasaoo 4 Claims.

This invention relates to processes of electrodepositing chromium, andaims to provide improvements therein.

The invention provides a novel process of de- 5 positing chromium, bywhich a good quality of plate, both as to structure, brightness, andcolor may be obtained, and with good efllciencies for chromium plating.

The invention further provides a process of electroplating chromiumwhich may be used without etching polished surfaces of articles to beplated.

According to the present invention, chromic acid is used as theelectrolyte. The catalyst is a compound of fluorine, usually a fluorid,such as chromic or sodium fluorid. We have obtained satisfactory brightplates using as catalysts the following: chromic fluorid, sodiumfluorid, fluosilicic acid, fluoboric acid (I'IBF4) and hydrofluoricacid, the substances being arranged substantially in their order ofefliciencies.

Temperatures from 15 C. to 35 C. are quite suitable, and currentdensities from $4 to 1 ampere per square inch are suitable, the lowercurrent densities being preferred for bright deposits.

The efliciency of the current in depositing chromium varies with theamount of fluorid used in.

the bath. With a solution containing 250 grams per liter of chromic acidthe best current efilciency has been found tobe around 3 grams per literof the fluorine compound, calculated as chromium fluorid, theefliciencies decreasing above and'below this amount, the fluorineradical being considered the constituent of importance. In amounts up toabout 10 grams per liter of the fluorid the solution does not etchpolished surfaces of articles (steel, iron, brass, for example) placedtherein for electroplating.

A mode of procedure is as followsz-A bath is made up containing 250grams per liter of chromic acid (Croz) and about 2 grams per liter ofhydrofluoboric acid (made by adding hydrofluoric acid to boricacid4I-IF+HaBO3- HBF4+3H2O) or an equivalent amount of a solublefluoboric compound. The article tobe plated, a polished piece offlatware (a spoon) for example, is connected to the cathode busbar and acurrent of. about ampere per square inch passed through thesolution tothe cathode at room temperature.

Good bright chromium plate generally is obtained at bath temperatures.15 to 35 C. The temperature may be kept approximately constant at adesired temperature by suitable heating and cooling means.

55 The process may be carried out by other modes of procedure than thatherein specifically described. As a guide to such other modes ofprocedure it may be stated that chromic acid baths containing thefluoboric compound follow the general rule of the catalyst compoundbeing. proportional to the concentration of chromic acid in the bath;and of the current-density increasing with temperature for maintaining agiven current efliciency.

In the present process, involving the use of fluoboric compounds we havefound that chromium plate is best obtained at a bath concentration of150 grams per liter chromic acid. In such a solution the fluoboric acidcontent may be 0.39 to 2 grams per liter, the optimum varying somewhatwith temperature, being approximately as follows:-

20 C.0.88 grams per liter fluoboric acid 30 C.-0.86 grams per literfluoboric acid 40 C.--1.15 grams per liter fluoboric acid 50 C.-1.43grams per liter fluoboric acid Mirror plate is best obtained by the useof baths of lower chromic acid concentration, as for example the 150gram per liter baths described above, with which mirror plate may beobtained through a considerable range of temperature and currentdensity, as for example- 20 C.-% amp/sq. inch 30 C. amp/sq. inch 40C.--% 'amp./sq. inch 50 C.-1 amp/sq. inch At 450 grams per liter ofchromic acid, chromium plates show a milkiness at 20 C. to 50 C. over awide range of current densities and fluoboric acid content. Y

The range of fluoboric content is not quite as wide as in the case ofthe chromium fluoride.

Subject-matter claimed in this-case has been divided from ourapplication, Serial No. 49,820, flied August 12, 1925, now Patent1,844,751, this application being a continuation in part thereof.

What is claimed is:-

' 1. A process of electroplating chromium comprising passing an electriccurrent through a solution containing chromic acid and a solublefluoboric compound, the amount of fluoboric compound being 3 grams perliter, or less of fluoboric acid for a solution containing 250grams perliter CrOa, or equivalent of other fluoboric compound, and aproportionately higher or lower fluoboric acid content for a higher orlower CrOa content.

2. A process of electroplating chromium comprising passing an electriccurrent through a solution containing chromic acid and a soluble nfluoboric compound. the amount of fluoboric com-.

4. A process according to claim 3, wherein the relations of thefollowing are substantially as follows:

Fluoboric acid Temperature Current density 0.86 grams per liter 20 C V4amp. per sq. in. 0.86 grams per liter 30 C M amp. per sq. in. 1.15 gramsper liter 40 C 96 amp. per sq. in. 1.43 grams per liter 5( C 1 amp. persq. m.

COLIN G. FINK. HUGH DAVID MCLEESE.

